Wireless sensors are highly desirable for many modern technological applications. These sensors have significant potential for use in a variety of fields, including environmental monitoring, health monitoring, monitoring of smart homes and businesses, monitoring of oil and gas exploration and pipelines, and monitoring in aerospace applications. Wireless sensors may be used for simple identification, measurement of environmental parameters, such as temperature or humidity, or provide information on parameters associated with more complex chemical and biological systems. According to a report published in October 2012 in IDTechEx, it is predicted that market value for wireless sensor network will exceeds $2 billion by the year 2022. [see F. Gonzalez and P. Harrop, IDTechEx, Batteries & Supercapacitors in Consumer Electronics 2013-2023: Forecasts, Opportunities, Innovation (October 2012)]
For many of these applications, wireless sensors and sensor networks afford the only access to critical information otherwise constrained by physical and/or environmental factors. Ironically, the fact that wireless sensors enable information gathering from difficult to access environments, for example, on moving objects, in secure areas, and in harsh environments, also makes it difficult to provide the sustainable power required for the use of active sensors. In fact, power consumption requirements for active wireless sensors is a limiting factor preventing the more widespread adoption of active sensors in certain wireless sensing applications. Consequently, passive wireless sensors, which operate without a battery or need for other external power source, enable a myriad of embedded sensor applications, and afford the only reasonable access to information from harsh and/or difficult to reach environments that might otherwise be constrained by physical and/or environmental factors. Unfortunately, passive sensors, by virtue of their design, are prone to significant interference being generated by non-target sensors. While several methods have been developed to partially mitigate this interference, none have achieved a satisfactory level of mitigation to allow for large networks of wireless passive sensors.